Piston ring and method of manufacturing the same

ABSTRACT

A manufacturing method of a piston ring is developed, wherein the method provides a countermeasure for wear of butt ends of the ion plating film coated piston ring and a countermeasure for breakage of the piston ring and the piston ring manufactured at a low cost. A thickness of the film ( 2 ) in the vicinity of butt ends ( 7 ) of a piston ring ( 1 ) is made greater than the thickness of the film ( 2 ) at other outer peripheral surface. Piston ring blanks ( 5 ) are rotated around their own axes toward an evaporation source ( 4 ) and a speed is lowered when the butt ends ( 7 ) face the evaporation source ( 4 ).

TECHNICAL FIELD

The present invention relates to a piston ring for an internalcombustion engine, and more particularly to a piston ring for aninternal combustion engine which exhibits an improvement on a film and athickness of an outer peripheral surface thereof and a method formanufacturing the same.

BACKGROUND ART

Recently, because of a demand for a higher power of an engine andmeasures for an exhaust gas to minimize atmospheric pollution, useenvironment of the piston ring is becoming severer and wear of an outerperipheral surface of the piston ring has been a problem to be solved.To cope with such the problem, there has been provided a piston ringwhich has an outer peripheral surface thereof coated with a hard filmwhich exhibits excellent anti-scuffing and wear resistance such as afilm made of chromium nitride, titanium nitride or the like by ionplating in place of a conventional hard chromium plating film ornitriding treatment.

However, in a diesel engine using a piston ring with such a nitride onan outer peripheral surface thereof as a hard film, when the combustionpressure is high or when an exhaust gas countermeasure is taken by alarge quantity of EGR (Exhaust Gas Recirculation), there arises theproblem that the wear on the outer peripheral slide surface isincreased. Accordingly, it is less than optimal to ensure the lifetimeand the sealing performance of the piston ring for a long time.Particularly, with respect to an outer peripheral slide surface of aportion of butt ends of a top ring which define a gap, since a hightemperature combustion gas passes a closed gap between the butt endswhen compressed within a cylinder and hence, the lubrication conditionis more serious compared to other portions. Further, since forcepressing the outer peripheral slide surface of piston ring to an innerwall surface of the cylinder by combustion gas pressure is greater atbutt ends than that at other portion of a piston ring, the wear of theouter peripheral surface at the butt ends is greater than that of theother surface.

Due to such wear, when the hard film formed on the outer peripheralslide surface of the butt ends or the portion thereof by ion plating isdissipated, a steel material which constitutes a substrate is exposed toa slide surface so that the wear rapidly progresses whereby the closedgap size between the butt ends is increased resulting in lowering of thesealing function.

Lowering of the sealing function attributed to the increase of closedgap size increases a quantity of blowby gas which is a combustion gasblowing to an oil pan side so that oil is degraded. Further, alubricating oil leaks to a combustion chamber side and this brings aboutnot only the increase of an oil consumption but also the occurrence ofenvironmental problem such as the increase of sulfuric component in anexhaust gas or the like. Further, when the hard film is dissipated dueto wear and the substrate material of steel is exposed, the danger ofengine scuffing is also increased.

In view of the above, finding of a countermeasure to cope with the wearin the vicinity of the butt ends has been an important task to besolved. As the countermeasure to cope with the wear of the butt ends, inJapanese Unexamined Patent Publication No. 2000-120866, there has beenproposed an idea in which a radial wall thickness in the vicinity of thebutt ends is set smaller than that of other portion of a piston ring byapproximately 20 to 30% and hence, a face pressure attributed to an ownexpanding force is decreased at the butt ends whereby the sealingability is maintained and, at the same time, the wear of the butt endsis prevented.

However, with respect to a top ring, at a top dead point of a piston,explosive combustion pressure is applied to a back surface of the pistonring and hence, the butt ends are pressed to an inner peripheral wall ofthe cylinder with a pressure which far exceeds a pressure derived fromthe own expanding force. Accordingly, it is the fact that the proposaldisclosed in the previously mentioned publication is also less thanoptimal as the countermeasure.

Further, in a high-load diesel engine which increases the combustionpressure or in an engine which adopts an exhaust brake, the piston ringis deformed in a dish shape at the time of explosive combustion or atthe time of operating an exhaust brake and this is liable to induce wearof ring grooves of the piston. When the ring groove of the piston isworn, the dish-like deformation of the piston ring is enlarged. Due tothis repeated deformation stress, the fatigue breakage of the pistonring is liable to be induced at a position opposite to the butt ends atwhich the stress is concentrated.

The hard film is poor in toughness and cracks are liable to easilyinitiate due to the stress concentration. Particularly, when a thicknessof the hard film is increased, this tendency becomes more remarkable.Further, when the hard film is made of nitride formed by ion plating(for example, chromium nitride, titanium nitride or nitrides in whichoxygen, carbon, boron or the like is added to those nitrides), there hasbeen a problem that the film is peeled off due to fatigue of repeatedfriction force which is attributed to the sliding movement between thepiston ring and an inner peripheral wall of the cylinder.

To prevent this film peeling caused by the frictional force, it iseffective to intentionally introduce a compressive residual stress intothe film by changing a bias voltage or the like in the ion platingprocess. In the ion plating, in general, the film has a tendency to growand to be stacked while generating a compressive stress in a filmforming process in reference to the substrate. However, when thecompressive residual stress is made to remain in the hard film, thetensile residual stress is generated in a portion of the substratematerial right below the film. When the piston ring is deformed in adish shape, provided that an external stress and the residual stress arecombined, a maximum tensile stress is generated right below the film inmany cases. When a defect or the like exists in the vicinity of theportion, it functions as a starting point of fatigue failure and hence,there arises a new problem that the breakage of the piston ring isliable to occur easily. This tendency becomes more apparentcorresponding to the increase of the thickness of the hard film.

Further, when the hard film is uniformly thick as mentioned previously,there arises a problem on a cost besides the above-mentioned problems.Particularly, when the hard film is formed by ion plating, ion platingequipment is expensive, the film forming speed is slow compared to thatof other method such as electric plating or thermal spraying and hence,the productivity is low. Further, a target such as chromium or titaniumis also expensive, the greater the film thickness, the piston ringbecomes more costly.

Further, the shape of the piston ring is changed when microstructure ofmaterial is changed. The deformation is liable to occur in the directionthat the curvature of the free shape of the butt ends of the piston ringis increased, that is, in the direction that the diameter of the pistonring is decreased.

Accordingly, film forming is performed at a temperature of approximately500° C. which is equal to or below a tempering temperature of thesubstrate material. However, as mentioned previously, when thecompressive stress remains in the film, the film is deformed even at atemperature which the microstructure change of the material does notoccur and this deformation is remarkably large when the heat-resistantelement in the substrate material is small in quantity. Accordingly,when a compressive residual stress is generated in the film, thistendency becomes particularly apparent along with the increase of thefilm thickness. Particularly, since the curvature of the outerperipheral portion in the vicinity of the butt ends is substantiallyequal to the nominal diameter of the piston ring, when the curvature ofthis portion increases (the curvature being continuously changed todecrease the diameter), a non-contact region is formed between the outerperipheral surface of the piston ring and the inner peripheral wall ofthe cylinder.

In the manufacturing process of the piston ring, there exists a lappingstep in which, to finish the piston ring into predetermined nominalouter diameter size and shape, and further to smooth the surfaceroughness of the outer periphery of the piston ring, the piston ring isset inside a lapping sleeve having a predetermined inner diameter afterperforming film forming by ion plating and the outer periphery surfacemade of the film is polished. In the piston ring coated with the ionplating film, due to the above-mentioned reason, the non-contact regionis liable to be easily formed on the outer peripheral portion in thevicinity of the butt ends in the lapping step. To obtain the completecontact between the outer peripheral portion of the piston ring and theinner peripheral wall of the lapping sleeve, it takes a considerabletime for lapping and, in a worst case, even when the other film isdissipated, the contact in the vicinity of the butt ends cannot beobtained.

Further, since an inner peripheral surface of the sleeve for lapping isworn out in that case, the lifetime of the sleeve for lapping isextremely shortened resulting in the manufacturing of costly pistonring.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above-mentionedproblems and it is a task of the present invention to provide a pistonring and a method for manufacturing the same which can achieve both ofthe measure to cope with wear of butt ends of the piston ring on which ahard film, especially, an ion plating film is formed on the outerperipheral surface and the measure to cope with breakage of the pistonring and can manufacture the piston ring at a low cost.

The present invention provides a piston ring having an outer peripheralsurface coated with a hard film, wherein the hard film holds acompression residual stress and a film thickness of the hard film in thevicinity of butt ends is set greater than that of other outer peripheralsurface.

It is preferable that the film thickness in the vicinity of the buttends is set 1.5 to 4 times as thick as that at a portion opposite to thebutt ends.

Further, it is preferable that the hard film is mainly composed ofnitride.

According to the present invention, there is further provided an ionplating film forming method for piston rings in which the stacked pistonring blanks are rotated around their own axes and revolved around anevaporation source of ion plating film, and a rotational speed of thestacked piston ring blanks at a point of time that the butt endsstraightly face the evaporation source or during a period ranging frombefore to after the butt ends straightly face the evaporation source isslower than a rotational speed when the other portion straightly facethe evaporation source.

According to the present invention, in a method for manufacturing theabove-mentioned piston rings according to the first or second invention,there is further provided a method for manufacturing piston rings,wherein the stacked piston ring blanks are mounted on a number of jigsand, at the same time, at the time of rotating around their own axes andrevolving around an evaporation source which is positioned at arevolving center so as to coat an ion plating film to outer peripheralsurfaces of the stacked piston ring blanks, the stacked piston ringblanks are set on the respective positions in a ion plating equipmentsuch that rotation of the butt ends of each stacked piston ring blanksis in phase and, thereafter, the rotation/revolution speeds of thestacked piston ring blanks when the butt ends straightly face theevaporation source are set slower than those of them when the otherportions straightly face the evaporation source.

Advantageous effects of the piston ring of the present invention are asfollows:

(1) The hard ion plating film having high wear resistance which iscoated to the outer periphery surface has the compressive residualstress and hence, irrespective of the presence of a frictional forcewhich is repeatedly loaded due to a sliding movement with an innerperipheral wall of a cylinder, it is possible to prevent peeling off thefilm attributed to fatigue.

(2) On the outer peripheral surface in the vicinity of the butt endswhich is liable to be worn due to the severe sliding condition, the hardfilm having a large thickness is formed and hence, it is possible toeliminate problems such as lowering of sealing performance or scuffingwhich are conventionally occurred due to the rapid wear progress by thedissipation of the hard film and the exposure of the substrate materialportion.

(3) The hard film has the smaller film thickness at portions other thanthe butt ends and hence, even when the compressive residual stress isheld in the film, chipping of the hard film or the like hardly occurswhereby the yield rate of the manufacturing process can be enhanced.Further, since the film is thin, a tensile residual stress whichgenerates in the substrate portion right below the film is alsodecreased and hence, the fatigue rupture of the piston ring is hardlyoccurred.

(4) Compared to a case in which a thick film is uniformly formed on theouter peripheral surface, only a thickness of the portion which isliable to be worn is increased and hence, the film forming time can beshortened whereby the coating productivity using the expensive ionplating equipment can be improved and the target consumption can be alsoreduced. Accordingly, it is possible to provide the piston ring at a lowcost.

(5) Corresponding to an amount of increase of the curvature of the outerperipheral portion of the butt ends which are generated by film forming,it is possible to increase the film thickness in the vicinity of thebutt ends compared to that of other portions and hence, in a lappingstep, it is possible to easily ensure a contact with the outerperipheral portion of the butt ends of the piston ring whereby thelapping productivity can be enhanced. Alternatively, since the wear of alapping sleeve can be reduced, it is possible to produce the piston ringat a low cost.

Next, the advantageous effects of the method for manufacturing thepiston ring are explained.

(1) By making the rotational speed of the stacked piston ring blanks ata point of time that the butt ends straightly face the evaporationsource or during a period ranging from before to after the point of timethat the butt ends straightly face the evaporation source slower thanthat the rotational speed of the stacked piston ring blanks at a pointof time that other portions straightly face the evaporation source, itis possible not only to set the film thickness in the vicinity of thebutt ends to an arbitrary thickness compared to the film thickness ofother portions but also to provide the film thickness which can bechanged at an arbitrary change rate and also continuously.

(2) When the stacked piston ring blanks are mounted on a number of jigsand they are rotated around their own axes and revolved around theevaporation source which is positioned at the revolving center so as tocoat ion plating films to outer peripheral surfaces of them, they areset on respective positions in the ion plating equipment such that therotation of the butt ends of the stacked piston ring blanks is in phase,and the rotation/revolution speeds of the stacked piston ring blankswhen the butt ends straightly face the evaporation source are set slowerthan the rotation/revolution speeds of the stacked piston ring blankswhen the other portions straightly face the evaporation source.Accordingly, it is possible to easily coat the hard film having thearbitrary film thickness distribution to the outer peripheral surfacesof the piston ring blanks. Further, the properties dispersion within onebatch processing can be reduced and hence, the piston rings can bemanufactured with high stability in quality. Accordingly, it is possibleto provide the piston rings at a low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a plan view of a piston ring as viewed in the axialdirection. In the drawing, A is an outer peripheral film thickness of aportion opposite to butt ends and B is an outer peripheral filmthickness in the vicinity of the butt ends. FIG. 1( b) is across-sectional view of the piston ring in the radial direction of aportion opposite to the butt ends. FIG. 1( c) is a cross-sectional viewof the piston ring in the radial direction in the vicinity of the buttends.

FIG. 2 is a plan view showing one embodiment of an ion plating equipmentfor manufacturing the piston rings of the present invention.

FIG. 3 is a graph showing rotational speed patterns of embodiments 1, 2,3.

FIG. 4 is a graph showing rotational speed patterns of embodiments 4, 5.

FIG. 5 is a schematic showing a fatigue testing method.

FIG. 6 is a graph showing fatigue limits of outer peripheral portions ofa comparison example and the embodiments 1 and 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention are explained indetail.

FIG. 1 shows a piston ring 1 which constitutes one example of thepresent invention. In the drawing, (a) is a plan view of the piston ringas viewed in the axial direction and (b) is a cross-sectional view ofthe piston ring 1 in the radial direction of a portion opposite to buttends, wherein A indicates a thickness of an ion plating film 2. (c) is across-sectional view of the piston ring 1 in the radial direction in thevicinity of the butt ends, wherein B indicates a thickness of the ionplating film 2.

A typical manufacturing process of the above-mentioned piston ring isexplained.

First of all, a steel wire or strip material capable of having arbitraryvarious cross-sectional shapes is formed into a coil of an approximatelypiston ring shape. Here, an outer peripheral surface shape may be also aBF (Barrel Faced), an eccentric BF or a half BF.

In coiling step, the steel wire material is coiled into a curvaturewhich is preliminarily obtained by correcting a change of curvaturegenerated by strain relieving heat treatment after coiling. This settingof curvature may change also depending on a shape of a cross-sectionalarea, a nominal diameter of a final ring and the alloy composition ofthe steel wire. Particularly, the influence of the alloy composition islarge. When the alloy composition is of high C % and contains acomponent such as Cr, Mo, V, Nb or the like which shows the highresistance against softening in tempering treatment, the change ofcurvature due to the strain relieving heat treatment after coiling issmall. The steel wire such as spring steel which contains a small amountof such an alloy component is characterized by exhibiting a large changeof curvature due to the strain relieving heat treatment after coiling.

Further, in coiling, the steel wire material is coiled one by one usingan NC control. However, the steel wire material may be formed one by oneusing a master cam having a given curvature in mass production in aprofiling system.

The piston ring material which is formed into a given outer peripheralcurvature in the above-mentioned manner is subjected to the strainrelieving heat treatment which conforms to the alloy composition so asto relieve a residual stress. Although the heat treatment conditionsdepend on the alloy composition of the steel wire, the steel wire isusually held at a temperature of 450 to 650° C. for 30 minutes to 1hour.

Thereafter, when necessary, gas nitriding, both side-surface grinding,outer peripheral surface grinding, and outer peripheral surface lappingare performed. Next, cleaning such as pickling (acid cleaning), alkalinedegreasing or the like is performed, and the stacked piston ring blanksare set on respective positions in an ion plating equipment.

An ion plating equipment and an ion plating method for forming a hardfilm are explained hereinafter.

FIG. 2 is a plan view showing the inside of the ion plating equipment 3and shows the relationship between the position of an evaporation source4 and the positions of stacked piston ring blanks 5. The stacked pistonring blanks 5 mounted on each jig such that butt ends of them arearranged in a fixed direction are arranged at 12 positions in a radialdirection around the evaporation source 4 at an equal angle. The stackedpiston ring blanks 5 revolve around the evaporation source 4 by using aplanetary gear structure not shown in the drawing and, at the same time,rotate around their own axes toward the evaporation source.

The rotation of stacked ring blanks is set such that each stacked pistonring blanks 5 performs the 215/17 rotation during a period in which itperforms 1 revolution around the evaporation source 4. The gear ratiobetween the revolution and the rotation is not specifically limited tothe above-mentioned value and there is no problem in choosing anarbitrary ratio. Since the rotation/revolution structure of theplanetary gear system is adopted, the equipment is configured such thatthe rotational shafts are rotated by way of gears at the above-mentionedgear ratio by rotating the revolution shaft.

The piston ring blanks 5 already cleaned are axially stacked on the jigssuch that the butt ends are arranged in a fixed direction. These jigsare set to 12 positions respectively. Here, to make rotation of allstacked piston ring blanks set on 12 positions be in phase, they are setsuch that the butt ends are directed toward the center of evaporationsource.

The inside of a vessel 6 is evacuated by a vacuum pump. When thepressure inside the vessel 6 is lowered to a value equal to or less thana given pressure, the piston ring blanks 5 are heated by a heater notshown in FIG. 2 to achieve a given temperature while being rotatedaround their own axes and revolved. Here, to ensure uniform heating, therotation/revolution speeds of respective shafts are fixed.

Next, film forming is started by running an arc to a known evaporationsource (target). Here, the film is formed by setting therotation/revolution speeds to fixed values such that the uniform filmthickness of approximately several μm is obtained. Thereafter, therotational speed of a motor not shown in FIG. 2 is controlled such thatthe rotational speed in the period that the butt ends of the piston ringis directed to the center of evaporation source is set to ½ to ¼ of therotational speed in other period.

Since the equipment of this embodiment adopts the combination of gearswhich provide one rotation for 17/215 revolution, the periodical speedcontrol is performed such that the 17/215 rotation of the revolutionshaft is set as 1 cycle and during this cycle, the rotational speed inthe period that the butt ends of the stacked piston ring blanks aredirected toward the evaporation source becomes the minimum rotationalspeed, and the speed is changed with the continuous sinusoidal waveformand the speed ratio becomes 2 to 4 times.

The butt ends of the stacked piston ring blanks 5 assume the minimumrotational speed in the period that the butt ends straightly face thecenter of evaporation source. Thereafter, as the butt ends are rotatedto the direction opposite to the evaporattion source by 180°, the speedis continuously increased and assumes the maximum speed at the directionopposite to the evaporation source by 180°. Again, the rotational speedis continuously decelerated such that it assumes the minimum speed atthe position where the butt ends straightly face the evaporation source.Accordingly, theoretically, the film thickness is continuously changedsuch that the film thickness at the butt ends assumes the maximum valueand the film thickness at the portion opposite to the butt ends by 180°assumes the minimum value. Further, the film is formed in the samemanner by adopting a method in which, the rotational speed of thestacked piston ring blanks is made slow in a fixed zone between pointsof time before and after the butt ends straightly face the evaporationsource.

The piston rings are cooled after film forming such that the innertemperature of the equipment assumes 100° C. and, thereafter, thepressure inside a vessel is returned to the atmospheric pressure and,thereafter, the jigs are removed. The piston rings are removed fromrespective jigs and the outer peripheral surfaces are lapped to a givensurface roughness using free abrasive grains in a lapping machine.Further, side surfaces are ground when necessary. Alternatively, in caseof the piston ring blanks 5 which are not subjected to nitridingtreatment, salt-bath nitriding or gas nitriding is performed andfinish-grinding of side surfaces is performed when necessary. Finally,grinding which adjusts gap sizes between the butt ends is performed soas to obtain given piston rings 1.

It is preferable that the piston ring size (diameter) is in a range of50 to 250 mmΦ, the thickness of the film is in a range of 0.01 to 0.1mm, and carbon steel, silicon chromium steel, or martensitic stainlesssteel is used as the piston ring material. It is preferable that thefilm thickness of an outer peripheral surface of the butt ends is 40 to55 μm.

Hereinafter, the explanation is made based on embodiments.

Using a steel wire having the alloy composition and heat treatmenthistory shown in table 1, having a rectangular cross section of radialwall thickness of 3.35 mm and ring width of 2.4 mm and also having anapproximately BF shape as a shape of a surface corresponding to theouter peripheral surface, the coiled material is formed using an NCcoiling machine with a curvature which is obtained by correcting thechange of curvature attributed to the strain relieving heat treatmentafter coiling and, thereafter, the piston ring blanks are produced bycutting the coiled materials one by one. Thereafter, the strainrelieving treatment, the both side-surface grinding, the gas nitridingtreatment and an outer peripheral surface lapping are performed. Next,these piston ring blanks are subjected to cleaning such as pickling,alkaline degreasing or the like so as to clean the outer peripheralsurface.

TABLE 1 Heat treatment steel Alloy composition, weight (%) conditionswire C Si Mn P S Cr Mo V Nb N Fe Hardening tempering A 0.85 0.30 0.350.02 0.02 17.50 1.05 0.11 — Balance 930° 630° B 0.65 0.30 0.35 0.02 0.0213.50 0.28 0.20 — Balance 930° 630° C 0.65 0.25 0.30 0.02 0.02 10.500.35 0.20 0.10 — Balance 1000° 625° D 0.60 0.25 0.20 0.02 0.02 8.05 0.330.15 — — Balance 980° 625° E 0.40 0.95 0.32 0.02 0.02 5.12 1.45 0.90 — —Balance 980° 625° F 0.65 1.43 0.67 0.02 0.02 1.58 0.30 0.58 — — Balance1000° 575° G 0.45 2.50 0.40 0.02 0.02 1.05 0.20 0.20 0.15 0.30 Balance980° 550° H 0.46 1.50 0.70 0.02 0.02 0.70 — — — — Balance 980° 450°

In the above-mentioned ion plating equipment 3 shown in FIG. 2, the jigseach of which axially stack the piston ring blanks 5 which are alreadysubjected to cleaning thereon such that the butt ends are arranged in afixed direction are set on 12 respective positions. Here, to make therotation of all stacked piston ring blanks 5 at 12 positions be inphase, the piston ring blanks 5 are arranged such that the butt ends aredirected to the center of evaporation source 4.

The inside of the vacuum vessel is evacuated to 2.5×10⁻⁴ torr using avacuum pump and the stacked piston ring blanks 5 are heated up to atemperature of 450° C. When the stacked piston ring blanks 5 reach agiven temperature and become stable, a nitrogen gas is introduced intothe vacuum vessel until the degree of vacuum reaches 1×10⁻² torr. Aftera lapse of 30 minutes, an operation mode is changed to change therotation/revolution speeds of the stacked piston ring blanks 5. Then, atthe rotational speed shown in Table 2 and using speed patterns shown inFIG. 3 and FIG. 4, an ion plating film is formed on outer peripheralsurfaces of the piston ring blanks after 7 hours and 30 minutes.

Metal chromium is used as the evaporation source 4 and coating isperformed at conditions where an arc current is set to 1000 A and a biasvoltage is set to 10V. The obtained film is CrN having hardness (HMV) of1800.

TABLE 2 Rotational speed of stacked piston ring blanks Embodiment 1Rotational speed is sinusoidal wave (maximum speed:minimum speed = 2:1),maximum speed = 6 turn/min Embodiment 2 Rotational speed is sinusoidalwave (maximum speed:minimum speed = 3:1), maximum speed = 6 turn/minEmbodiment 3 Rotational speed is sinusoidal wave (maximum speed:minimumspeed = 4:1), maximum speed = 6 turn/min Embodiment 4 Rotational speedin azone covering 45° before and after the butt ends straight face theevaporation source is 1/2 of speed at other portion, maximum speed = 6turn/min Embodiment 5 Rotational speed in a zone covering 45° before andafter the butt ends straight face the evaporation source is 1/3 of speedat other portion, maximum speed = 6 turn/min

After forming the film, cooling is effected until the temperature insidethe equipment becomes 100° C. and the pressure inside the vessel isreturned to atmospheric pressure. Thereafter, the jigs are removed and,then, the piston rings are removed from the jigs. Lapping is performedusing a lapping machine so as to lap outer peripheral surfaces of pistonrings with free abrasive grains under conditions shown in Table 3.

TABLE 3 Abrasive Material SiC Abrasive Grain Size #4000 Concentration ofAbrasive Grains 25 g/litter Speed  1 stroke/second

The film thickness of the outer peripheral surface at the butt ends andat the portion opposite to the butt ends coated under respectiveexperimental conditions and the numbers of lapping until a contact withthe outer peripheral surface is obtained with respect to each steel wireare shown in Table 4. Further, as a comparison example, a piston ringwhich is manufactured using a conventional ion plating film coatingmethod is shown. In this method, the rotation speed is fixed to 6turn/min.

According to the result of experiment using an X-ray diffraction, theresidual stress of the CrN film is 600 to 700 MPa with respect to allspecimens. After completion of lapping, finish grinding is performed toboth side surfaces of the piston rings and, finally, grinding isperformed so as to adjust a gap size between the butt ends, wherebygiven piston rings having a nominal diameter (d1) of 95 mm, a radialwall thickness (a1) of 3.35 mm and a ring width (h1) of 2.33 mm areobtained.

TABLE 4 Film thickness of Number of lapping (times) Film thickness ofportion opposite to Steel Steel Steel Steel Steel Steel Steel Steel buttends butt ends wire A wire B wire C wire D wire E wire F wire G wire HComparison 30 μm 32 μm 1000 1100 1300 1400 1500 2000 2000 NG exampleEmbodiment 1 41 μm 22 μm 300 300 350 350 350 500 700 800 2 50 μm 18 μm250 250 300 300 300 400 500 600 3 55 μm 15 μm 300 250 250 250 250 300330 500 4 44 μm 22 μm 300 275 350 350 350 450 500 600 5 52 μm 18 μm 250250 300 300 300 400 400 500 NG: A film at other portion disappearsbefore the contact is obtained over the whole periphery. Although thereexist cases in which, first of all, the contact is obtained at the buttends and, thereafter, the contact is obtained at other portions, thenumber of lapping until the contact is obtained over the whole peripheryis counted.

As shown in Table 4, in all kinds of steels, the piston rings of thepresent invention exhibit the small number of lapping until the contactwith the whole periphery is obtained compared to the conventional pistonring and hence, it is understood that the advantageous effect of thepresent invention is remarkable. Further, although it is estimated thata proper value exists with respect to the difference in film thicknessbetween the butt ends and the other portion based on the heat resistanceof steels, it is apparent that the rotational speed ratio can bedetermined by repeating the number of experiments.

Then, the fatigue test is performed on the manufactured piston rings.

As shown in FIG. 5, the fatigue tests are performed on the piston rings1 of the embodiments 1, 4 and the comparison example using a methodwhich repeatedly opens and closes the butt ends 7 of the piston ring 1.Results of the test are shown in FIG. 6.

FIG. 6 shows the fatigue strength of the outer peripheral portions ofthe piston rings of the comparison example and the embodiments 1 and 4.From the embodiments 1 and 4, it is evident that the fatigue strength isenhanced in both embodiments compared to the comparison example. Thepiston ring according to the present invention is effective forprevention of breakage attributed to fatigue at a position opposite tothe butt ends.

Further, the piston ring 1 of the comparison example using the steelwire A and the piston rings 1 of the embodiments 1, 3, 4 are set torespective cylinders of a 4-cylinder diesel engine and the durabilityevaluation is performed at a rated output for 600 hours. Results of theevaluation are shown in Table 5.

TABLE 5 Film thickness Film thickness of portion Expected of butt endsopposite to butt worn-out Worn-out after ends after time of outerportion durability test durability test peripheral film (expected) Com-12 μm 25 μm 1000 Hr butt ends parison example Embodi- ments 1 23 μm 17μm 1370 Hr butt ends 3 35 μm  9 μm 1500 Hr Portion opposite to butt ends4 24 μm 15 μm 1320 Hr butt ends

As can be understood from this table, in the piston rings of the presentinvention, the ion plating film thickness of the butt ends afterdurability test is greater than the corresponding ion plating filmthickness of other portion and hence, it is appreciated that the timeuntil the film is worn out is prolonged compared to the conventionalpiston ring. That is, it is understood that the durability of the pistonring is enhanced.

As described above, it is understood from FIG. 6 and Table 5 thatcompared to the conventional piston ring, the piston ring of the presentinvention exhibits excellent durability and has no risk of breakagethereof.

The advantageous effects of the piston ring of the present invention areas follows:

The hard ion plating film having high wear resistance which is coated tothe outer periphery of the piston ring holds the compressive residualstress and hence, there is no risk that the film is peeled off due tofatigue even when the piston ring receives the friction force which isrepeatedly loaded due to the sliding movement relative to the inner wallof the cylinder.

Further, the film having a large film thickness is formed on the outerperipheral surface in the vicinity of the butt ends where the slidingcondition is severe so that the film is liable to be worn out.Accordingly, it is possible to eliminate conventional problems such aslowering of the sealing performance and scuffing which is generated bythe rapid wear progress due to the exposure of the substrate materialportion after the dissipation of the hard film. Since the hard filmhaving a small film thickness is formed at portions other than the buttends, even when the film is made to hold the compressive residual stresstherein, the chipping of the hard film or the like hardly occurs so thatthe yield rate of the manufacturing process can be enhanced. Further,since the film is thin, the tensile residual stress which is generatedin the substrate material portion right below the film also becomessmall so that the fatigue rapture of the piston ring hardly occurs.

Compared to a case in which the thick film is uniformly formed on theouter periphery surface, the film is made thick only at portions whichare liable to be worn out and hence, the film forming time can beshortened whereby the productivity using the expensive film formingequipment can be enhanced and the target consumption is also small.Accordingly, the piston ring can be manufactured at a low cost.

Further, from a viewpoint of manufacturing, when the stacked piston ringblanks are mounted on a number of jigs and they are rotated around theirown axes and revolved around the evaporation source which is positionedat the revolving center so as to coat ion plating films to outerperipheral surfaces of the piston rings, they are set on respectivepositions in the ion plating equipment such that the rotation of thebutt ends of them is in phase, and the rotation/revolution speeds in theperiod that the butt ends straightly face the evaporation source are setlower than those in the period that the other portions straightly facethe evaporation source. By means of such a simple control, it ispossible to easily form the hard film having the arbitrary filmthickness distribution to the outer peripheral surfaces of the stackedpiston ring blanks.

Further, the method of the present invention exhibits the excellent massproductivity in one batch processing and, at the same time, theproperties dispersion of the piston rings are also small and hence, itis possible to ensure the high stability in quality. The piston ringwith the ion plating film coated by the above-mentioned process exhibitsthe small change of curvature at the outer peripheral portion of thebutt ends, and hence, the lapping time can be shortened and the wear ofthe lapping sleeve can be reduced.

Although the film thickness is inspected only at two points of the buttends and the portion opposite to the butt ends in these embodiments, inview of the fact of the rotational speed control like sinusoidal wavesand the state of contact surfaces at the time of performing lapping, itis evident that the film thickness is changed substantiallycontinuously.

Further, although the state of the contact surface at the time oflapping is slightly worse with respect to the piston ring which ismanufactured by making the rotational speed of the butt ends slow onlyin the vicinity of evaporation source compared to the case of thesinusoidal waves control, the level does not cause any serious problemfrom a manufacturing viewpoint.

In these embodiments, the piston rings are formed one by one in anelliptical shape using an NC controlled coiling machine. However, it isalso possible to obtain piston rings having the butt ends whose filmthickness is greater than that of other portions by steps as follows:

cutting the steel wire after continuously forming it in a perfectcircle;

widening a gap between the butt ends;

heating the blanks under the above-mentioned strain relieving heattreatment conditions and holding such heat treatment temperature so asto form an approximate shape; and, thereafter,

forming the outer peripheral surface into a given shape; and

performing post steps similar to those of these embodiments.

1. A piston ring using a piston ring blank having a fixed ring thicknesswhich is formed by coiling a steel wire material and having an outerperipheral surface thereof coated with a hard film by ion plating,wherein a film thickness of the hard film in the vicinity of butt endsis greater than a film thickness of other outer peripheral surfacewherein the hard film is a film which is mainly composed of nitride andholds a compressive residual stress therein.
 2. A piston ring accordingto claim 1, wherein the film thickness in the vicinity of the butt endsis 1.5 to 4 times as thick as the film thickness at a portion oppositeto the butt ends.
 3. A piston ring according to claim 1, wherein thefilm thickness of the outer peripheral surface of the butt ends of thepiston ring is 40 to 55 μm.